1. Field of the Invention
The present invention relates to a water-decomposable absorbent article for pantiliners, sanitary napkins, incontinence pads, diapers, etc.
2. Description of the Related Art
Recently, water-decomposable absorbent articles disposable in flush toilets have come available, including, for example, pantiliners, sanitary napkins, incontinence pads, diapers, etc. Such a water-decomposable absorbent article comprises a water-decomposable top sheet to face the skin of a wearer (i.e., the top sheet serves as a body facing surface), a water-decomposable back sheet to face an external support such as an underwear (i.e., the back sheet serves as a garment facing surface), and a water-decomposable absorbent core sandwiched between the top sheet and the back sheet. In general, the back sheet is provided with an adhesive layer via which the absorbent article is fitted to the external support such as an underwear, and the adhesive layer is covered with a release sheet so as not to degrade its adhesiveness until use. Before use, the release sheet is peeled off to expose the adhesive layer outside, and the absorbent article is fitted to the external support via the thus-exposed adhesive layer.
Water-decomposable absorbent articles have self-sustaining strength enough to keep their shape during use, and, after used, they are disposed of in toilets and readily decomposed in water. However, the release sheet which is to be peeled off before use, is made of a resin sheet or-a cardboard sheet coated thick with silicone, and is very troublesome since it must be treated as wastes. If the release sheet is also made from a water-decomposable material, it can be disposed of in toilets without requiring treating as wastes, and thereby being convenient to users.
In that situation, Japanese Unexamined Patent Publication (Kokai) No. Heisei 11-47187 discloses a water-decomposable individual packaging absorbent article, for which is used a water-decomposable release sheet. The water-decomposable release sheet used for it is made of a water-soluble resin film or a water-soluble paper coated with a silicone or Teflon for release treatment, and the absorbent article is individually wrapped with the release sheet.
However, the water-soluble resin film coated with a silicone or Teflon for release treatment is problematic in that, when it is stored for a long period of time in high-humidity environments, the film absorbs water and swells, and, as a result, the strength of the wrapping bag of the film is lowered. Another problem with it is that the swollen film will cause surface defects such as pin holes through the release treatment portion of silicone or the like. Such surf-ace defects tend to zip the release sheet when the release sheet is released from the adhesive layer of the absorbent article, and, as a result, smoothly releasing the release sheet from the adhesive layer will be often difficult. In case where the water-soluble resin film is made thick to increase its wet strength, such a thick film will gel in water when disposed of in flush toilets, and will clog drainpipes. The water-soluble paper coated with the lubricant (silicone) is also problematic in that the hydrophobic, three-dimensionally crosslinking release layer of silicone or Teflon will bond fibers of the water-soluble paper to each other, and, as a result, the release sheet will be poorly or hardly decomposable in water.
Japanese Unexamined Patent Publication (Kokai) No. Heisei 8-48950 discloses a release sheet prepared by sizing a water-decomposable paper with a water-soluble sizing agent followed by coating a surface thereof with silicone. However, the invention disclosed in this patent publication is essentially directed to a technique of recycling used paper for regenerated paper, and not to a technique of ensuring rapid decomposition in water of silicone-coated water-decomposable paper and of preventing the surface defects of the coating film of silicone. Japanese Unexamined Patent Publication (Kokai) No. Heisei 6-218006 discloses a water-degradable release liner prepared by coating water-degradable paper with a solution of polyvinylpyrrolidone, drying it, and further coating it with a lubricant silicone layer. In the disclosed release liner, however, the polymer material of polyvinylpyrrolidone is not heat-sealable. Therefore, when the release liner is tried to individually wrap absorbent articles with it, it cannot be formed into bags through heat-sealing via polyvinylpyrrolidone.
In those disclosed in the above-mentioned Japanese Unexamined Patent Publication (Kokai) Nos. Heisei 6-218006 and 8-48950, a water-soluble resin is infiltrated into the fiber-to-fiber space in water-decomposable paper, or water-decomposable paper is coated with a water-soluble resin solution thereby to size the paper. However, the composite film of water-decomposable paper and a water-soluble resin, which is produced by infiltrating the water-soluble resin into water-decomposable paper or by coating water-decomposable paper with the water-soluble resin solution in the manner disclosed, is still problematic in that a surface of the composite film tends to be roughened owing to the roughness of the constituent fibers. In case where a release layer of silicone or the like is formed on such a rough surface of the composite film, it could not have a uniform thickness since the underlying composite film does not have a flat surface. As a result, when the water-soluble resin in the composite film has absorbed water to swell, the release layer that overlies the composite film will have surface defects such as pinholes. Still another problem with the composite film produced by coating water-decomposable paper with the water-soluble resin solution is that the composite film is often warped in the step of drying it after coated with the resin solution, owing to the difference in the wet expansion coefficient between the water-decomposable paper and the water-soluble resin. As a result, when a release layer of crosslinked silicone or the like is formed on such a warped composite film, it will be cracked or will have surface defects such as pin holes.
Through the surface defects (cracks and pin holes) of the release layer, water will penetrate into the underlying water-soluble resin layer to swell or gel it, and the thus-swollen or gelled resin layer will be exposed outside through the surface defects of the release layer. If so, the release properties of the release layer for facilitating easy release of the release sheet from an adhesive layer will be thereby lowered, and, as a result, smoothly peeling the release sheet from the adhesive layer of an absorbent article will be difficult.
An object of the invention is to provide a water-decomposable absorbent article provided with an adhesive layer to which is fitted a water-decomposable release sheet, wherein the release sheet is improved so that its release layer is flat and is resistant to water, that it is well decomposable in water and that its releasability from the adhesive layer is surely kept for a long period of time.
Another object of the invention is to provide such a water-decomposable absorbent article, in which the release sheet can be formed into a bag for packaging the absorbent article and it can be heat-sealed via a water-soluble polymer layer that underlies the release layer.
According to one aspect of the invention, a water-decomposable absorbent article may comprise: an absorbent body that includes a water-decomposable top sheet, a water-decomposable back sheet, a water-decomposable adsorbent core sandwiched between the top sheet and the back sheet, and an adhesive layer provided on the back sheet to fit the absorbent article to an external support; and a water-decomposable release sheet for protecting the adhesive layer before use with covering the adhesive layer, wherein;
the release sheet includes a release layer that faces the adhesive layer, a water-decomposable fibrous layer, and a water-soluble polymer layer sandwiched between the release layer and the fibrous layer,
a basis weight of the release layer is at least 0.05 g/m2 but smaller than 0.3 g/m2, and an equilibrium contact angle between a surface of the release layer and a water drop thereon is at least 100 degrees.
Preferably, the release layer is made from a silicone compound layer, and its weight is at most 0.15 g/m2.
The release sheet for the water-decomposable absorbent article of the invention comprises a release layer of silicone or the like formed on a water-decomposable fibrous layer of, for example, water-decomposable paper or water-decomposable non-woven fabric, via a water-soluble polymer layer therebetween. In this, therefore, the lubricant constituting the release layer does not penetrate into the fiber-to-fiber space in the fibrous layer to bond the constituent fibers, and the decomposability in water of the release sheet is not lowered. When the release sheet is disposed of in flush toilets, water penetrates into the water-soluble polymer layer existing between the fibrous layer and the release layer, to thereby dissolve the polymer layer, and, after all, the release layer is readily peeled off from the thus-dissolved polymer layer in water jets.
In the invention, the weight of the release layer is specifically defined to fall within the range as above, and, in addition, the equilibrium contact angle between the surface of the release layer and a water drop thereon is specifically defined to be at least 100 degrees. Being so defined, the release layer is flat and is resistant to water. Therefore, even when the release sheet is stored under a high-humidity condition for a long period of time or water adheres thereto in the process of producing it, or while its release layer is kept in contact with a water-soluble adhesive, water hardly penetrates into its water-soluble polymer layer via its release layer. As a result, the water-soluble polymer that constitutes the polymer layer is prevented from being swollen or gelled by water, as little water penetrates into the layer, and the release layer is hardly damaged to have surface defects such as pin hole. For these reasons, the release sheet ensures good releasability from the adhesive layer for a long period of time. In addition, the release layer is readily dispersed in water, as the uppermost limit of its weight is defined as set forth above.
Preferably, the thickness of the water-soluble polymer layer falls between 5 xcexcm and 20 xcexcm. Also preferably, the water-soluble polymer layer is formed from a polyvinyl alcohol, and the polyvinyl alcohol for the layer has a degree of saponification of at least 70%.
In case where such a water-soluble polymer layer of which the thickness falls within the defined range is formed on the water-decomposable fibrous layer, the influence of the surface roughness of the fibrous layer on the polymer layer can be reduced than conventionally, and the release layer can be well formed on the polymer layer in a more stable condition. In addition, the polymer layer of which the thickness falls within the defined range can reduce the stress to warp the release sheet.
Also preferably, the release sheet is to form a bag to entirely wrap and envelop the absorbent article therein, and its outer periphery is heat-sealed via the polyvinyl alcohol layer to form the bag.
According to another aspect of the invention, the method for producing the water-decomposable absorbent article that includes a water-decomposable top sheet, a water-decomposable back sheet, a water-decomposable absorbent core sandwiched between the top sheet and the back sheet, and an adhesive layer provided on the back sheet to fit the absorbent body to an external support, may comprise;
a step of melt-extruding and laminating a water-soluble polymer layer having a thickness of from 5 xcexcm to 20 xcexcm on a surface of a water-decomposable fibrous sheet made of hydrophilic and water-insoluble fibers,
a step of applying a release material onto a surface of the water-soluble polymer layer to form thereon a release layer having a weight of at least 0.05 g/m2 but smaller than 0.3 g/m2,
a step of crosslinking the release material such that an equilibrium contact angle between the surface of the release layer and a water drop thereon is at least 100 degrees, thereby forming a release sheet, and
a step of fitting the release sheet to the adhesive layer of the absorbent body via the release layer.
In the method, the water-soluble polymer layer is melt-extruded and laminated on the fibrous layer. In this, therefore, it is easy to define the thickness of the polymer layer to fall within the desired range.